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Lecture 5: Strings(Sections 8.1, 8.2, 8.4, 8.5,
1st paragraph of 8.9)
CS 1110
Introduction to Computing Using
Python
[E. Andersen, A. Bracy, D. Fan, D. Gries, L. Lee,
S. Marschner, C. Van Loan, W. White]
http://www.cs.cornell.edu/courses/cs1110/2021sp
Announcements
• Did you try the 6 questions at the end of the slides from
the previous lecture? Check answers on course website
and ask at office/consulting hrs if you have questions!
• Want to find an assignment partner?
Make/answer a post on Ed Discussion megathread “[Where
can I] find a partner for this class?”
Learning Strategy Center (LSC) has a study partner finding
service
Talk to classmates in lab!
• Check out “Assignment Advice” on course website
• Note “Policies” on the course website
Today
• More about the str type
New ways to use strings
• More examples of functions
Functions with strings!
• Learn the difference between print and return
5
Strings are Indexed (Question 1)
• s = 'abc d'
• Access characters with []
s[0] is 'a'
s[4] is 'd'
s[5] causes an error
s[0:2] is 'ab' (excludes c)
s[2:] is 'c d'
• Called “string slicing”
• t = 'Hello all'
• What is t[3:6]?
6
a b c d
0 1 2 3 4
H e l l o
0 1 2 3 4 5
a
6
l
7
l
8
A: 'lo a'
B: 'lo'
C: 'lo '
D: 'o '
E: I do not know
Strings are Indexed (Question 2)
• s = 'abc d'
• Access characters with []
s[0] is 'a'
s[4] is 'd'
s[5] causes an error
s[0:2] is 'ab' (excludes c)
s[2:] is 'c d'
• Called “string slicing”
• t = 'Hello all'
• What is t[:3]?
8
a b c d
0 1 2 3 4
H e l l o
0 1 2 3 4 5
a
6
l
7
l
8
A: 'all'
B: 'l'
C: 'Hel'
D: Error!
E: I do not know
Other Things We Can Do With Strings
Operator in: s1 in s2
• Tests if s1 “a part of”
(or a substring of) s2
• Evaluates to a bool
Examples:
>>> s = 'abracadabra'
>>> 'a' in s
True
>>> 'cad' in s
True
>>> 'foo' in s
False
Built-in Function len: len(s)
Value is # of chars in s
Evaluates to an int
Examples:
>>> s = 'abracadabra’
>>> len(s)
11
>>> len(s[1:5])
4
>>> s[1:len(s)-1]
'bracadabr'
>>>
10
Defining a String Function
Want to write function
middle, which returns the
middle 3rd of a string (length
divisible by 3).
How we want it to behave:
>>> middle('abc')
'b'
>>> middle('aabbcc')
'bb'
>>> middle('aaabbbccc')
'bbb'11
Important Questions:
1. What are the parameters?
2. What is the return value?
3. What goes in the body?
def middle(text):
???
return middle_third
Steps to writing a program
1. Work an instance yourself
2. Write down exactly what you just did
3. Generalize your steps from 2
4. Test your steps
5. Translate to Code
6. Test program
7. Debug (if necessary)
12
Steps to writing a program
1. Work an instance yourself
2. Write down exactly what you just did
3. Generalize your steps from 2
4. Test your steps
5. Translate to Code
>>> middle('abc')
>>> middle('aabbcc')
>>> middle('It was the best of times, it was the worst of times, it was the age of
wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of
incredulity, it was the season of Light, it was the season of Darkness, it was the spring of
hope, it was the winter of despair, we had everything before us, we had nothing before us,
we were all going direct to Heaven, we were all going direct the other way…') 13
middle_third = text[2:4]
middle_third = text[1] Too easy!!
Still too easy!!
Definition of middle
def middle(text):"""Returns: middle 3rd of textParam text: a string with length divisible by 3"""
14
IMPORTANT:Precondition requires
that arguments to middle have length
divisible by 3.
If not? Bad things could happen, and we
blame the user (not the author) of the
function.
Advanced String Features: Method Calls
• Strings have some useful methods
Like functions, but “with a string in front”
• Format: <string name>.<method name>(x,y,…)
• Example: upper() returns an upper case version
>>> s = 'Hello World’
>>> s.upper()
'HELLO WORLD’
>>> s
'Hello World’
16
>>> s[1:5].upper()
'ELLO'
>>> ‘scream'.upper()
‘SCREAM'
>>> 'cs1110'.upper()
'CS1110'
Examples of String Methods
• s1.index(s2)
Returns position of the first
instance of s2 in s1
error if s2 is not in s1
• s1.count(s2)
Returns number of times s2
appears inside of s1
• s.strip()
Returns a copy of s with
white-space removed at ends
• s = 'abracadabra’
• s.index('a')
• s.index('rac')
• s.count('a')
• s.count('b')
• s.count('x')
• ' a b '.strip()
17See Python Docs for more
0
2
5
2
0
'a b'
a b r a c
0 1 2 3 4
a
5
d
6
a
7
b
8
r
9
a
10
String Extraction Example
def firstparens(text):
"""Returns: substring in ()
Uses the first set of parens
Param text: a string with ()"""
>>> s = 'One (Two) Three'
>>> firstparens(s)
'Two'
>>> t = '(A) B (C) D'
>>> firstparens(t)
'A'
18
Steps to writing a program
1. Work an instance yourself
2. Write down exactly what you just did
3. Generalize your steps from 2
4. Test your steps
5. Translate to Code
6. Test program
7. Debug (if necessary)
20
Think of all the corner cases
What could possibly go wrong?
String Extraction Puzzle
def second(thelist):"""Returns: second word in a listof words separated by commas, with any leading or trailing spaces from the second word removedEx: second('A, B, C') => 'B'Param thelist: a list of words withat least two commas """
start = thelist.index(',')
tail = thelist[start+1:]
end = tail.index(',')
result = tail[:end]
return result
Is there an error?
A: Yes, Line 1
B: Yes, Line 2
C: Yes, Line 3
D: Yes, Line 4
E: There is no error23
1
2
3
4
5
Not All Functions Need a Return
def greet(n):
"""Prints a greeting to the name n
Parameter n: name to greet
Precondition: n is a string"""
print('Hello '+n+'!')
print('How are you?')
28
Displays these
strings on the
screen
No assignments or return
(returns None)
print vs. return
• Displays a value on screen
• Used primarily for testing
• Sends a value from a function
call frame back to the caller
• Important for calculations
• Does not display anything
29
def print_plus(n):
print(n+1)
>>> print_plus(2)
3
>>>
def return_plus(n):
return n+1
>>> return_plus(2)
3
>>>
?
unexpected printing courtesy of:
Python Interactive Mode
• executes both statements and expressions
• if expression: 1. evaluates2. prints value (if one exists)
>>> 2+2
4
>>> return_plus(2)
3
>>>30
evaluates (performs addition)
prints value (4)
evaluates (makes function call, gets return value)prints value (3)
>>> return_plus(2)3>>>
return_plus 1
return_plus in action
31
def return_plus(n):return n+1
call framen 2
Python Interactive Mode
RETURN 3
1. Evaluates : makes function call, evaluates to return value
2. Python interactive mode prints that value
1
>>> print_plus(2)3>>>
print_plus 1
print_plus in action
32
def print_plus(n):print(n+1)
call framen 2
Python Interactive Mode
RETURN NONE
1. Evaluates : • makes function call• prints (executes line 1)• return value is NONE
2. does not print that value because it’s NONE
1
>>> print_plus(2)23>>>
print_plus 1
hybrid_plus in action
33
def hybrid_plus(n):print(n)return n+1
call framen 2
Python Interactive Mode
RETURN 3
1. Evaluates : makes function call, evaluates to return value
2. Python interactive mode prints that returned value
2
1
2
See the difference in the Python Tutor
def print_plus(n):
print(n+1)
def return_plus(n):
return n+1
x1 = print_plus(2)
x2 = return_plus(2)
print(x1)
print(x2)
34http://cs1110.cs.cornell.edu/tutor/#mode=edit
Exercise 1
Module Text
# module.py
def foo(x):
x = 1+2
x = 3*x
Python Interactive Mode
>>> import module
>>> print(module.x)
…
35
A: 9
B: 10
C: 1
D: None
E: Error
What does Python
give me?
Exercise 2
Module Text
# module.py
def foo(x):
x = 1+2
x = 3*x
y = foo(0)
Python Interactive Mode
>>> import module
>>> print(module.y)
…
37
A: 9
B: 10
C: 1
D: None
E: Error
What does Python
give me?
Exercise 3
Module Text
# module.py
def foo(x):
x = 1+2
x = 3*x
return x+1
y = foo(0)
Python Interactive Mode
>>> import module
>>> module.y
…
39
A: 9
B: 10
C: 1
D: None
E: Error
What does Python
give me?